The present application relates to the detection and tracking of aircraft. The invention finds particular application in detecting and tracking suitably equipped aircraft on an airport surface, aircraft on closely spaced parallel approaches and aircraft in proximity of an airport and will be described with reference thereto. It is to be appreciated however, that the teachings disclosed herein are also amenable to the detection and tracking of suitably equipped vehicles in a variety of environments such as highway, maritime and other applications.
The position of aircraft near airports and other aircraft is of crucial importance to the safe movement of aircraft Multiple techniques which make use of the Air Traffic Control Radar Beacon System (ATCRBS) transponders have previously been employed to track and monitor airborne aircraft enroute to an airport, provide guidance or monitor aircraft on final approach to airports and monitor aircraft movement within the airports runways and taxiways. The tracking techniques have been realized in the following systems.
Airborne aircraft enroute to an airport are monitored using Air Traffic Control Beacon Interrogator (ATCBI-6) ground systems and associated displays. These systems interrogate Mode A/C and are fully compliant with the new Mode S (for selective) transponder formats that include P4 suppression to reduce synchronous garble.
Traffic Collision And Avoidance System (TCAS) is operated onboard aircraft to interrogate aircraft transponders and measure time of arrival (TOA) and in some cases angle of arrival (AOA), then track display and issue resolution advisories when aircraft closure criteria exceed certain thresholds, as taught In U.S. Pat. No. 5,387,915 issued on 7 Feb., 1995 to Moussa et el.
The Transponder Landing System (TLS) provide means for the pilot to monitor aircraft position with respect to the desired course and glide path, on final approach to an airport. The system relies on Mode A/C interrogations to track the transponder.
Systems are known which monitor aircraft on the surface of an airport based only on time of arrival multilateration of the aircraft transponder response, and some prototype multilateration systems have been fielded which rely heavily on the new Mode S transponder, as taught in U.S. Pat. No. 5,262,784 issued on 16 Nov., 1993 to Drobnicki et el.
The fundamental design constraint of all of the above systems for which the transponder reply is the basis of positive identification is the desire for the system to operate during periods of synchronous garble. Garbling of transponder replies occurs where positive identification of the aircraft transponder response is thwarted by overlapping transponder replies from aircraft that are near the same slant distance from the ground sensor. To increase the capacity of the transponder based systems and minimize synchronous garble, the new Mode S has been defined and standardized. All aircraft with more than 30 seats are now equipped with Mode S transponders and TCAS. In contrast to normal Secondary Surveillance Radar (SSR), some features of the SSR Mode S make it very suitable for ground traffic control as well. The problems in cooperative ground detection tracking methods are due to the lack of Mode S transponders in general aviation aircraft. This problem is likely to persist for many years. For reliably detecting aircraft within dense RF environments which include Mode A/C transponders, new methods must be developed.
The present invention provides an improved method and apparatus for measuring and processing aircraft transponder replies degraded by synchronous garble which overcomes problems with the current techniques used by the above-referenced systems and others.
In accordance with one embodiment of the present invention, a method of tracking aircraft in a surveillance area includes a conventional IFF transponder responsive to interrogation signals at a first frequency, receiving reply signals from aircraft in the surveillance area at a second frequency, the signals being received on a plurality of antenna arrays. An angle of the received reply signals is determined relative to each array from the differential carrier phase. A range is determined based on the time of arrival (TOA) measurement, which is the time from signal transmission to reception. These and other characteristics of the reply signals, such as amplitude and frequency, are used to correlate pulses with each other over time. A position is calculated for each reply pulse indicative of an origin or source point of the reply signals from data including the determined angle and range. This Pulse Track data is conveyed to a central processing location. Pulse tracks are then correlated between multiple ground sensors using all track states which include reply amplitude, position (point of reply origin), velocity, acceleration and reply frequency (waveform cycles per second), to yield an aircraft position and ID. A single array may not be able to calculate a unique angle, but TOA measurements from multiple antenna arrays can be used to calculate a unique angle. This is accomplished by using the TOA data from 2 non-co-located arrays to calculate an initial position and then selecting the angle with the lowest residual. Further processing of the data allows the ultimate system accuracy to be achieved by solving for aircraft position estimates based on intersecting two or more lines of bearing. This final step in the transponder reply processing eliminates the transponder encoding delay. Data representative of a plurality of calculated positions is periodically provided to vehicles in the surveillance area or other users such as ground or air traffic controllers.
In accordance with another aspect of the present invention, a system for locating an object within a monitored area which receives and measures at least two angles from at least two separate sensor locations without external interrogation of the transponder, commonly referred to as a transponder Mode S squitter.
In accordance with another aspect of the present invention, Angle of arrival data is measured on every reply pulse to improve pulse to pulse correlation. A position can be calculated for each pulse and combined with velocity, acceleration, reply amplitude and reply signal frequency, can be used to associate pulses over time. This provides synchronous garble mitigation, as the entire reply is not needed to provide position and ID.
In accordance with an aspect of the present invention, the angle is determined by determining at least one of an elevation angle and an azimuth angle.
In accordance with an aspect of the present invention, the method further includes receiving the provided data representative of a plurality of calculated positions in a particular vehicle, for example, an aircraft either airborne or taxiing, or airport service vehicles. From the received data a position corresponding to the particular vehicle is extracted and compared with another position from onboard sensors.
In accordance with an aspect of the present invention, the angle of each reply pulse is determined by receiving the reply signal on a reference antenna in the array and receiving the reply signal on another antenna in the array. A difference in phase between the signal received on the reference antenna and the signal received on the other antenna is determined.
In accordance with an aspect of the present invention, the method of tracking reply pulses in a surveillance area further includes calculating an elapsed time between transmitting an interrogation signal and receiving the reply signal and determining a range based on the elapsed time.
In accordance with an embodiment of the present invention, an apparatus which detects aircraft in an area and includes an angle determining apparatus, a range determining apparatus, a position processor and a transmitter. The angle determining apparatus includes a plurality of antennas disposed as an array, and a phase calculator which calculates a difference in phase of the reply signal between a first receive channel including a first antenna, and a second receive channel including a second antenna. The range determining apparatus includes a synchronized timer which determines a time between the interrogation signal and receipt of the transponder reply signal at each of the plurality of antennas, and a range estimator which estimates a range based on the determined time between the interrogation signal and receipt of the transponder reply signal. The position processor determines a position based on the calculated difference in phase and the estimated range. The transmitter transmits data including positions representative of a plurality of aircraft in the area.
In accordance with an aspect of the present invention, the apparatus also includes a second angle determining apparatus in data communication with the position processor which determines a position based on the calculated difference in phase and the second calculated difference in phase.
In accordance with an embodiment of the present invention, a system for locating a cooperative object within a monitored area includes an interrogator which transmits an interrogation signal at a frequency. First and second arrays receive a reply signal transmitted by a transponder. An angle of arrival processor calculates an angle of the received reply signal relative to the first and second arrays, and a position processor calculates a position based on data including the calculated angle of the received reply signal.
In accordance with an aspect of the present invention, the system further includes a display in communication with the position processor which graphically depicts the calculated position.
In accordance with an aspect of the present invention, the system further includes an identification processor in communication with the sensor which determines an identification associated with the object based upon information encoded in the transponder reply signal.
In accordance with an aspect of the present invention, the system further includes a range processor which determines a range of the transponder from the ground station based on the received reply signal.
In accordance with an aspect of the present invention, the system further includes means for otherwise detecting objects in the monitored area and displaying detected objects simultaneously.
In accordance with an aspect of the present invention, the number of arrays is at least two, and the arrays are non-linearly disposed.
In accordance with an aspect of the present invention, the system further includes a broadcaster which broadcasts a plurality of calculated positions throughout the monitored area.
In accordance with another embodiment of the present invention, a system for locating a vehicle within a monitored area, includes at least one receiver array which receives a reply signal transmitted by a transponder associated with the vehicle. A range processor calculates a range based on the received reply signal and an angle of arrival processor calculates an angle of the received reply signal. A position processor calculates a vehicle position based upon the range and the angle.
In accordance with an aspect of the present invention, the range processor includes a time of arrival processor which calculates an elapsed time between a reference time and a time the reply signal is received at the receiver array, where the range is determined from the elapsed time at the receiver.
In accordance with an aspect of the present invention, the reference time includes receipt of a timing signal at the receiver.
In accordance with an aspect of the present invention, the angle of arrival processor extracts an in phase (I) component and a quadrature (Q) component of the signal, and further calculates an azimuth based on the in phase and quadrature components.
In accordance with an aspect of the present invention, the receiver array includes two adjacent closely spaced antenna elements to provide a coarse azimuth calculation, and two non-adjacent antenna elements linearly disposed with the adjacent elements to provide a fine azimuth calculation.
In accordance with another embodiment of the present invention, a method of determining a position of a cooperative object in a monitored area, includes receiving an encoded reply signal at a frequency from the cooperative object transmitted in response to an interrogation signal at another frequency. A line of bearing is calculated from characteristics of the received reply signal, the line of bearing relative to a plurality of antenna elements forming an array. A second solution is calculated based on receipt of the reply signal, and a position is determined based on the line of bearing and the second solution.
In accordance with another aspect of the present invention, the characteristics of the received reply signal from which the line of bearing is calculated include a spatial phase distribution of the reply signal.
One advantage of the present invention resides in the use of legacy equipment to provide a more reliable and accurate navigation solution during synchronous garble events.
Another advantage of the present invention resides in the use of AOA data to provide the most efficient use of each interrogation and or transponder reply in an environment where it is extremely important to minimize interrogations, and as a consequence minimize the occurrence of garble.
Another advantage of the present invention resides in the increased situational awareness of both pilots and ground-based controllers.
Another advantage of the present invention resides in the application of the present teachings to any transponder based navigation applications such as TCAS, Parallel Runway Monitoring, TLS, or airborne and ground surveillance.
Still further advantages will become apparent to those of ordinary skill in the art upon reading and understanding and following detailed description.